Firewall
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Firewall — nftables / ufw
The Linux firewall: filtering traffic with simple ufw and low-level nftables, checking the rules.
6 minufw allowufw statusufw default -
The classic iptables firewall
The classic Linux firewall — chains, rules, NAT, and why nftables replaced it.
5 miniptables -Liptables -Aiptables -I -
nmap: which ports are visible from outside
A port scanner: shows which ports of a machine are open, closed or filtered, as another machine sees them.
8 minnmapnmap -pnmap -p- -
conntrack — connection tracking
The kernel's connection table — what NAT/firewall see, overflow, debugging.
4 minconntrack -Lconntrack -Econntrack -S